Studies of age-related changes in thymic function and reconstitution after bone marrow transplant (BMT) nave focused on the epithelial component of the thymic microenvironment; little attention has been paid to the influence of the endothelial compartment of the thymus in this process. The goal of this proposal is to delineate the role of the thymic vascular niche in the regulation of homing and engraftment of bone marrow cells to thymus after transplantation. Clinical data from patients with Severe Combined Immune Deficiency (SCID) and our own data in experimental immune deficient murine models, show that when non-conditioned BMT is performed specifically in the neonatal period, rapid thymic engraftment results in donor lymphopoiesis, with little or no contribution of donor cells to non-lymphoid lineages. These findings demonstrate that although microenvironmental signals for T cell differentiation exist throughout postnatal life, different host mechanisms for thymic engraftment exist in neonatal and adult hosts. We propose that in the neonatal setting, bone marrow can home to and engraft the thymus directly without the need for an intermediate stage of marrow engraftment. Further, we hypothesize that the presence of immature, Vascular Endothelial Growth Factor (VEGF) responsive, vasculature in the neonatal thymus provides the niche that mediates the process of efficient and rapid thymic engraftment. In Specific Aims 1 and 2, we will use the striking dichotomy between neonatal and adult thymic engraftment patterns to define the endothelial mechanisms regulating these differences. Data from murine allogeneic models will be translated to the human setting by testing our findings in a similar xenogeneic BMT model and by analysis of human thymus samples at different stages of postnatal development. In Specific Aim 3, we will explore the cross-talk that exists between the vascular niche, thymocytes and thymic epithelium to control the process of homing and engraftment. Defining the mechanisms by which T cells are produced in the neonatal thymus after nonconditioned BMT. will allow us to develop targeted and effective cell therapies for patients with SCID and for other clinical settings plagued by the problems of delayed or incomplete immune reconstitution.